The invention relates to a fuel injection pump for internal combustion engines as generically defined hereinafter.
In such fuel injection pumps the high-pressure or positive-displacement chamber, or pump work chamber, is filled with fuel from the pump interior, which is a low-pressure chamber, via the uncovered control opening in the cylinder wall during the intake stroke of the pump piston, which is driven by a rotating control cam. The supply stroke begins as soon as the pump piston control edge has moved past the control opening and the control opening is closed by the pump piston jacket face. Since the entire stroke of the pump piston is always constant, the quantity of fuel pumped for injection from the high-pressure chamber upon each piston stroke can be varied by adjusting the usable supply stroke. This is done via the recess with the diversion edge, which defines the jacket face of the piston toward the recess. As soon as the diversion edge begins to slide via the control opening, the control opening communicates to an increasing extent with the high-pressure chamber, via the recess. Since the supply pressure is substantially higher than the pressure in the low-pressure chamber or pump interior, the fuel flows out of the high-pressure chamber suddenly, via the control opening, into the low-pressure chamber. The fuel supply is thus shut off instantly. By rotating the pump piston about its longitudinal axis, the instant at which the oblique or helical diversion edge slides past the control opening--and hence the end of supply--can be adjusted to later or earlier, with respect to the stroke period of the pump piston.
Because of the considerable pressure difference between the supply pressure in the high-pressure chamber and the fuel pressure in the low-pressure chamber, the outflow of the fuel via the control opening is so intensive upon the diversion, or in other words upon termination of the supply stroke, that eddy flows and negative-pressure zones are created at the control opening, which lead to cavitation at the piston jacket face. The formation of vapor bubbles can particularly be observed in the diversion process, and particularly when they implode they cause considerable damage to the control surface of the piston.
To diminish these phenomena, in a known fuel injection pump of this generic type (German Patent No. 27 49 693) the line or bore in the cylinder jacket connecting the control opening with the low-pressure chamber is divided into at least one feed branch and at least one outflow branch, and upon the initiation of the diversion process a connection is established between the feed branch and the outflow branch by means of the groove, which is closed at the end, in the piston face between the control edge and the diversion edge. As a result, fuel is supplied to the outflow branch from the feed branch during the diversion process, and as a result the eddy currents and negative-pressure zones are largely counteracted, consequently reducing the forces of cavitation.
In another known fuel injection pump (U.S. Pat. No. 4,163,634), the groove disposed between the control edge and the diversion edge in the pump piston jacket face serves not to prevent cavitation damage but rather to reduce the fuel supply quantity when the engine is under load and its rpm is accordingly dropping. To this end, the groove extends over part of the diversion edge and leaves open the part of the diversion edge that is operative during idling, or in other words that at the idling setting of the pump piston corresponds with the control opening. With one end, the groove is connected to the high-pressure chamber, and its other end is closed. It is disposed relative to the diversion edge in such a way, and is dimensioned in such a way that from a particular point on the supply stroke until the end of the supply stroke, it is located in the vicinity of the control opening and allows some of the fuel in the high-pressure chamber to flow out via the control opening. The groove is inoperative during idling of the fuel injection pump.